C5-C6 annelated naphthopyrans are photochromic compounds capable of changing colors under the influence of a poly or monochromatic light such as, for example, UV light. The compounds then return to their initial color when the irradiation ceases, or under the influence of temperature and/or a poly- or monochromatic light different from the initial light. C5-C6 annelated naphthopyrans find applications in various fields such as, for example, in the manufacture of ophthalmic lenses, contact lenses, solar protection glasses, filters, camera optics or other optical devices and observation devices, glazing, and decorative objects. 2H-chromenes annelated at C5-C6 in some cases possess a neutral gray or brown color upon UV irradiation, which is particularly interesting when used in photochromic lenses since it does not require the use of a mixture of dyes of different colors to obtain the desired tint. Indeed, dyes of different colors may have different resistance to UV aging, different fading kinetics, or different thermal dependence, which leads to a change in the tint of the lens during its use. For example, in the case of ophthalmic lenses, it is highly desirable that the photochromic article exhibit fast bleaching in the absence of UV light, both for the comfort of vision and for safety reasons (e.g., during driving).
The synthesis of C5-C6 annelated naphthopyrans has been previously described. One approach involves the reaction of a R-substituted 1-phenyl-3,4-dihydro-1H-naphthalene-2-one with cyanoethylacetate, followed by intramolecular Friedel-Craft cyclization to yield a cyano substituted naphthol. Depending on the R substituent, the yield of this reaction may be very low, which ultimately increases the purification time and manufacture costs of the naphthopyran. Moreover, the process involves high temperatures. For example, the Friedel Craft cyclization is carried out at 200° C., and the removal of the cyano group involves elevated temperatures (220° C.), which is problematic with many temperature sensitive substituents such as, for example, electron-withdrawing groups, present on the naphthopyran. Such substituents may be useful in tuning the photochromic properties of the dye. Therefore, it would be desirable to have a synthetic procedure for the preparation of 2H-chromenes annelated at C5-C6 that require less stringent reaction conditions and provide increased yields.